It has to do with how the liver processes it since first starts there, since B6 is a slow processed, if you have an fair amount i.e about 10-25% of the RDI, it will be processed immediately into 5-HT, which doesn't cross the blood-brain barrier, but does cross into all the areas with serotonin, the heart, the intestines etc.
This is depending how your body is a good or bad thing, some people get Irritable bowels if they take too much 5-HTP, some people get them fixed by it. Nonetheless, most of it's effects are probably placebo if you consume your B6 RDI everyday, though L-tryptophan isn't as picky in conversion

And has a far easier time being able to convert into 5-HT and the various chemicals that surround it's production. It is far easier to be regulated by the body than 5-HTP which only has one choice, then another, or just pissed out, most of being of the last.
There are various reports (Not just Shulgin here) that have FELT like they were on some strong opiates from orange juice that had extra sugar. =)
Tryptophan break down>
(Not made by me, but check out the picture)
As we can see L-Tryptophan clearly has a larger span of various other chemicals in the body. On top of that, it doesn't all metabolize in the liver, it has amino acid production in several places (Namely, one very important one, THE BRAIN!)
I only did this out of love and giving people the correct information.
PEACE OUT YA'LL

5-HTP is the chemical precursor to serotonin, a critical
neurotransmitter (among a few lesser functions) that is at present time
believed to be connected to a wide array of emotions and behaviors
ranging from depression to OCD. Research suggests that supplemented
5-HTP both contributes to and stimulates serotonin production, however
there is at this writing little conclusive evidence on the
effectiveness of 5-HTP as a treatment for things like depression or
'E-hangovers' (MDMA markedly depletes serotonin levels in the brain).
Taking 5-HTP by itself should have no immediately recognizable
psychoactive effects, but that's not to say it is in any way a placebo.
Taking too much 5-HTP, especially before or while doing drugs that
inhibit the metabolism or stimulate the production or concentration of
serotonin may contribute to a potentially fatal condition called serotonin syndrome.

Not being a medical professional, I recommend 100-300mg 5-HTP daily
for three to five days after E-binging to alleviate the symptoms of
serotonin deficiency. Even if it is just Power of Suggestion, it's not
hurting any worse than the E did ;)

Now, here's the definition of placebo:

A substance containing no medication and prescribed or given to reinforce a patient's expectation to get well.

An inactive substance or preparation used as a control in an
experiment or test to determine the effectiveness of a medicinal drug.

Something of no intrinsic remedial value that is used to appease or reassure another.

5-HTP is an active medication that is prescribed or given to help a patient get well that also has the potential to harm if misused. The write-up you linked to states:

...without Carbidopa, more than a few milligrams of extra B6 per day would be expected to insure that most dietary 5-HTP gets turned into serotonin before it can get into the brain.
...
At the very best, people who take B-vitamins with 5-HTP, or who take 5-HTP products with B6, waste their money.

You seem to be asserting everyone who uses 5-HTP is on an abnormal or supplemented diet. You have a point - which under specific conditions may be valid, but you're not proving anything wrong :(

Not really, most people that would taking any amount of nootropics or Amino acids usually take Vitamin supplements as well, not to mention most people eat rich food of it,. There are currently several drugs that barely surpass the placebo (SSRIs being the most common one at 60-65% efficient ;) ). Also I did mention that it's a slow metabolization just like every other B-vitaminn (It also stores in the LIVER FOO) (B-12 if taken orally, can take upto 400 DAYS until it really gets into your system), it slowly takes it's time to work into your system. Thats just one comment, there are various other information about B6 and the devolopment of serotonin in many nootropics and nutrition books.
Also, it's very difficult to reach Serotonin Syndrome by 5-HTP alone since, your body is not stupid, and will just piss out most of what you don't need. Also if you start really reading up on your nootropics, you start to notice that unless you're B6 deficienct, it seems extremely likely that in fact it is placebo. 50/50 chance, go brain go.
Trust me, you do feel a difference between L-Tryptophan and 5-HTP.

"Not really, most people that would taking any amount of nootropics or
Amino acids usually take Vitamin supplements as well, not to mention
most people eat rich food of it,."

I disagree on both counts. A study involving ~5,500 older american male participants showed an average intake of about 1.5mg b6 per day, while the DRI is 2-2.5mg per day. Furthermore all but maybe two people who regularly drop E that I've met in my travels even care about preventative/protective measures let alone take a multivitamin. If you are going to be taking any sort of supplement it's expected you're to read into it and ensure it does not react poorly with anything else you're taking; that's just common sense.

Doing amphetamines on opium doesn't make opium a placebo, it makes you an idiot.

My next point is that nothing sits in the liver. Shit is always passing though, being partially metabolized, then sent back out one step closer to 'purity'. I'd like to know how there can be enough B6 in the liver to effectively and completely neutralize an unnatural dose of 5-HTP on the _first pass_?

Is isn't as the amount that really matters, in fact most of the 5-HTP that has B6 only has the RDI of a single daily use, it's more having it there, then anything else, it has a lot to do with L-tryptophan and it's processes. Since 5-HTP only has one option to transform, and the majority of serotonin is used in the body itself, it almost metabolized instantly for that purpose, also has a lot to do with the processes of L-Tryptophan, 5-HTP is unheard of as a basic amino acid in foods, L-Tryptophan is not, since it as such, and how L-tryptophan metabolizes, it most likely confuse the body and as such, it probably metabolize fully into 5-HT just like that, the majority of 5-HTP (if any at all) would seem very unlikely that it would be possible for it to really reach the brain, it also co-relates to the amount of people that I know take it and feel an effect (About 50%). I have known a lot of people that try to measures against ecstasy and other drugs, I'm surprised your list is so small
As my as we'd both like to be right or wrong, we don't know squat about our bodies (Though they're really educated guesses) As well as most people that I have insisted they give L-tryptophan a chance, have indeed thought it to be comparing night and day. Maybe I helped pushed that effect along I don't know but I have gotten about 10 people sworn by it over 5-HTP.... I could babble about this all day, BUT I GOT A SANDWICH!

Is isn't as the amount that really matters, in fact most of the 5-HTP that has B6 only has the RDI of a single daily use,

Indeed, I've never seen a 5-HTP supplement in Canada that contains B6 while I was looking for 5-HTP and doubt I'd make the mistake of buying one if I ever came across it. Regardless, I was talking about 5-HTP supplements, not 5-HTP with B6. You can't just add things to my statements :p

As my as we'd both like to be right or wrong, we don't know squat about our bodies

That's actually my point; I never made any assertions and took great care to point out that research on 5-HTP and its effects is incomplete. You however set out to "prove me wrong" and make me "correct" myself, all you've done is made a fool of yourself :(